The proposed investigation is based on the evidence recently obtained that an immunizing infection in mice with Listeria monocytogenes besides resulting in a short-lived state of active immunity that is mediated by replicating T cells, also results in a long- term state of heightened resistance to reinfection that depends on the persistance of small population non-replicating sensitized T cells. The cellular basis of this long-lived state of immunological memory will be studied further. Particular attention will be given to determining whether the cells that carry this form of T cell memory belong to the pool of recirculating lymphocytes, and whether they are the same cells that are responsible for the long-lived state of delayed sensitivity that follows immunizing infection with Listeria. This will involve attempts to collect the cells responsible for both states of immunological reactivity from thoracic duct lymph. It will also involve attempts to separate the cells responsible for these 2 states by subjecting cell populations from sensitized donors to affinity chromotography and other methods of cell fractionation. The in vivo immunological reactivity of the cell fractions will be tested by infusing them into normal recipient mice. The final goal of the investigation is to find an adaptive function for delayed hypersensitivity.